Semiconductor switching devices are used for inverter control in railways or in electric power transmission. When a plurality of semiconductor switching devices connected in parallel are operated, current imbalance arises due to dispersion in characteristics between the devices.
FIG. 4 is a diagram illustrating turn-on switching waveforms of two semiconductor switching devices connected in parallel. FIG. 5 is a diagram illustrating turn-off switching waveforms of two semiconductor switching devices connected in parallel. I1 and I2 denote currents flowing through the two semiconductor switching devices, VGE denotes a gate-emitter voltage, and VCE denotes a collector-emitter voltage. It is apparent from FIGS. 4 and 5 that imbalance arises between the currents I1 and I2.